1. Field of the Invention
The present invention relates to a so-called reflective type liquid crystal display device used in word processor, personal computer, other office automatic appliances, view finder of portable video tape recorder, and various monitors of video signals.
2. Description of the Related Art
The EL (electro-luminescence), CRT (cathode ray tube), and LED (light emitting diode) are display devices which emit light by themselves, while the liquid crystal is a display device which does not emit light by itself but receives light and exhibits a display. Therefore, it requires a light source in order that the display be visualized so as to be visible by the human eye. Hitherto, many structures have been proposed and realized for the light source device of direct-viewing type liquid crystal display devices. Principal examples are given below.
(a) Illumination lamp type
FIG. 1 is a sectional view showing a constituent example of a liquid crystal display device using a light source device of illumination lamp type. Lamps 11a, 11b are disposed on both sides of the front side of a liquid crystal display device 12. The light from the lamps 11a, 11b passes through the liquid crystal display device 12, and is reflected by a reflector 13, and is projected again to the liquid crystal display device 12 to be display light. In the case of the illumination lamp type, the lamps 11a, 11b as the light source may be installed at the front side of the display surface of the liquid crystal display device 12, and the number of parts is small, and a simple and inexpensive liquid crystal display device service is presented.
(b) Reflector mirror type
FIG. 2 is a diagram showing a constituent example of a liquid crystal display device using a light source of reflector mirror type. In the reflector mirror type, the light utilization efficiency is high, and a high luminance is obtained, and hence it is widely employed. On the opposite side of a liquid crystal display device 24 of a lamp 23, a reflector 22 is disposed, and the light from the lamp 23 is efficiently radiated to the front side (the liquid crystal display device 24 side). By the reflector 22 alone, the high luminance portion is deviated to the periphery of the lamp 23, and it tends to be uneven in luminance, and therefore a diffusion plate 21 is disposed in front of the lamp 23 and the luminance uniformity is improved by varying the thickness of the diffusion plate 21. The light from the diffusion plate 21 is projected to the liquid crystal display device 24.
(c) Flat plate lamp type
FIG. 3 is a diagram showing the constitution of a liquid crystal display device using a light source device of flat plate lamp type. A fluorescent compound is applied to the both inner surface of a front glass plate 35 and a rear glass plate 36, and a fluorescent plane 31 is formed. At both right and left ends of the fluorescent plane 31, discharge electrodes 32a, 32b are disposed, and by discharge between the discharge electrodes 32, 32b, the fluorescent plane 31 emits light. The light from the fluorescent plane 31 is projected to a liquid crystal display device 37. In this flat plate lamp type, the lamp itself is a flat plate, which may be disposed at the rear side of the liquid crystal display device 37, and the optical system is not necessary, so that the efficiency of utilization of light is high.
(d) Light guide plate type
FIG. 4 is a diagram for showing the constitution of a liquid crystal display device using a light source device of light guide plate type. The light radiated from a lamp 41 is guided by multiple reflection of the inner surface of a light guide plate 43 composed of acrylic resin excellent in light transmissivity. A reflector 42 is disposed on the opposite side surface of a liquid crystal display device 45 of the light guide plate 43, and the light from the lamp 41 is taken out through a diffusion plate 44 only from the front side, and is projected to the liquid crystal display device 45. Here, the lamp 41 focuses the light by making use of the reflector 42 and a slit not shown, and the utilization efficiency of light is improved, but since this light source device is not designed to make use of total reflection of the light guide plate 43 in its principle, the reflector 42 and the slit are not for limiting the incident angle of the light. This light source device is relatively thin, and is excellent in uniformity of luminance, so that it is applicable to thin design of electronic appliances utilizing portable liquid crystal display devices.
(e) EL type
The EL is a thin, lightweight, flat light source device, excellent in uniformity of luminance, possessing properties as the light source device for liquid crystal display device, but it is low in surface luminance, narrow in selection width of light color, and fast in color deterioration during use, and it has been replaced by the fluorescent lamp in the color trend of liquid crystal display device. Recently, however, the EL of high luminance and long life is being developed, and the EL lamp is being reviewed in the background of the thin design of liquid crystal display device.
(f) Transparent reflector type
FIG. 5 is a diagram showing the constitution of a liquid crystal device using a light source device of transparent reflector type. The light radiated from a lamp 51 is reflected by a front reflector 54 disposed at the front side (the observer 55 side) of a liquid crystal display device 52, and passes through the liquid crystal display device 52, and is reflected by a rear reflector 53, and passes through the liquid crystal display device 52 again and then the front reflector 54, and reaches the observer 55 looking at the liquid crystal display device 52. The liquid crystal display device making use of this light source device has not been realized yet.
Recently, the word processor, personal computer, and other office automation appliances are becoming small and portable progressively. In the portable appliance, considering the convenience of carrying, thin size and light weight are essential conditions, and the keyboard, display device and battery are rapidly becoming thinner in size and lighter in weight. On the other hand, reduction of power consumption is also important, and the reflection type liquid crystal display device shows the display that is visible only with external light in the sufficiently illuminated environments, and the display device without light source device is widely employed. In this type of liquid crystal display device, however, the display is hard to read when the ambient illumination becomes dark, which often causes troubles in use.
To solve these problems, what is needed is the reflective type liquid crystal display device with a light source device capable of illuminating the entire surface of the display device uniformly, being light in weight and thin in size. In the reflective type liquid crystal display device, since it is not possible to illuminate from the rear side, a transparent light source device must be disposed in front of the display surface. The reflective liquid crystal display device having a transparent prepositional light source device presents a display visible with external light only without using light source device built in the appliance as far as the ambient illumination is bright enough, and the light source device built in the appliance may be used when the ambient illumination is insufficient, and thus the light source device may be used only when required, so that the power consumption may be saved.
Of these six types of prior art above, in the (b) reflector mirror type, (c) flat plate lamp type, and (e) EL type, structurally, the light source device cannot be disposed before the liquid crystal display device. The (d) light guide plate type has the reflector, which cannot be place at the front because the light source is not transparent.
Both (a) illumination lamp type and (b) transparent reflector type can be both installed before the liquid crystal display device, but uniform illumination is difficult. In the (f) transparent reflector type, the light source device becomes large and thick. In addition, in the display mode using polarizer, such as TN-LC (twisted nematic liquid crystal) and STN-LC (super-twisted nematic LC), the liquid crystal molecules in the liquid crystal display element is twisted by 90 to 270 degrees as initial orientation, and the liquid crystal display elements are disposed between a pair of polarizers, end display is made by making use of the optical properties of the liquid crystal display elements, that is, the optical rotation characteristics without electric field and optical rotation cancellation characteristics in voltage applied state, but in the (a) illumination lamp type or (f) transparent reflector type, since the light source device must be disposed outside of the two polarizers, the light source light passes through each polarizer twice, or four times in total. Accordingly, the light absorption by the polarizer is large, and the utilization efficiency of light source light is lowered, and the display becomes dark.